Antigen-presenting cells (APCs) can induce tolerance or immunity. We describe a subset of human APCs that express indoleamine 2,3-dioxygenase (IDO) and inhibit T cell proliferation in vitro. IDO-positive APCs constituted a discrete subset identified by coexpression of the cell-surface markers CD123 and CCR6. In the dendritic cell (DC) lineage, IDO-mediated suppressor activity was present in fully mature as well as immature CD123+ DCs. IDO+ DCs could also be readily detected in vivo, which suggests that these cells may represent a regulatory subset of APCs in humans.

Pharmacological inhibition of indoleamine 2,3-dioxygenase (IDO) activity during murine gestation results in fetal allograft rejection and blocks the ability of murine CD8+ dendritic cells to suppress delayed-type hypersensitivity responses to tumor-associated peptide Ags. These observations suggest that cells expressing IDO inhibit T cell responses in vivo. To directly evaluate the hypothesis that cells expressing IDO inhibit T cell responses, we prepared IDO-transfected cell lines and transgenic mice overexpressing IDO and assessed allogeneic T cell responses in vitro and in vivo. T cells cocultured with IDO-transfected cells did not proliferate but expressed activation markers. The potency of allogeneic T cell responses was reduced significantly when mice were preimmunized with IDO-transfected cells. In addition, adoptive transfer of alloreactive donor T cells yielded reduced numbers of donor T cells when injected into IDO-transgenic recipient mice. These outcomes suggest that genetically enhanced IDO activity inhibited T cell proliferation in vitro and in vivo. Genetic manipulation of IDO activity may be of therapeutic utility in suppressing undesirable T cell responses.

BackgroundIndoleamine 2,3-dioxygenase (IDO) is an enzyme with immune-suppressive properties that is commonly exploited by tumors to evade immune destruction. Anti-tumor T cell responses can be initiated in solid tumors, but are immediately suppressed by compensatory upregulation of immunological checkpoints, including IDO. In addition to these known effects on the adaptive immune system, we previously showed widespread, T cell-dependent complement deposition during allogeneic fetal rejection upon maternal treatment with IDO-blockade. We hypothesized that IDO protects glioblastoma from the full effects of chemo-radiation therapy by preventing vascular activation and complement-dependent tumor destruction.MethodsTo test this hypothesis, we utilized a syngeneic orthotopic glioblastoma model in which GL261 glioblastoma tumor cells were stereotactically implanted into the right frontal lobes of syngeneic mice. These mice were treated with IDO-blocking drugs in combination with chemotherapy and radiation therapy.ResultsPharmacologic inhibition of IDO synergized with chemo-radiation therapy to prolong survival in mice bearing intracranial glioblastoma tumors. We now show that pharmacologic or genetic inhibition of IDO allowed chemo-radiation to trigger widespread complement deposition at sites of tumor growth. Chemotherapy treatment alone resulted in collections of perivascular leukocytes within tumors, but no complement deposition. Adding IDO-blockade led to upregulation of VCAM-1 on vascular endothelium within the tumor microenvironment, and further adding radiation in the presence of IDO-blockade led to widespread deposition of complement. Mice genetically deficient in complement component C3 lost all of the synergistic effects of IDO-blockade on chemo-radiation-induced survival.ConclusionsTogether these findings identify a novel mechanistic link between IDO and complement, and implicate complement as a major downstream effector mechanism for the beneficial effect of IDO-blockade after chemo-radiation therapy. We speculate that this represents a fundamental pathway by which the tumor regulates intratumoral vascular activation and protects itself from immune-mediated tumor destruction.

Hemophagocytic lymphohistiocytosis (HLH) is an inborn disorder of immune regulation caused by mutations affecting perforin-dependent cytotoxicity. Defects of this pathway impair negative feedback between cytotoxic lymphocytes and APCs, leading to prolonged and pathologic activation of T cells. Etoposide, a widely used chemotherapeutic drug which inhibits topoisomerase II, is the mainstay of treatment for HLH, though its therapeutic mechanism remains unknown. We utilized a murine model of HLH, involving lymphocytic choriomeningitis virus infection of perforin deficient mice to study the activity and mechanism of etoposide for treating HLH and found that it substantially alleviated all symptoms of murine HLH and allowed prolonged survival. This therapeutic effect was relatively unique among chemotherapeutic agents tested, suggesting distinctive effects on the immune response. We found that the therapeutic mechanism of etoposide in this model system involved potent deletion of activated T cells and efficient suppression of inflammatory cytokine production. This effect was remarkably selective; etoposide did not exert a direct anti-inflammatory effect on macrophages or dendritic cells and it did not cause deletion of quiescent naive or memory T cells. Finally, etoposide’s immunomodulatory effects were similar in wild type and perforin deficient animals. Thus, etoposide treats HLH by selectively eliminating pathologic, activated T cells and may have utility as a novel immune modulator in a broad array of immunopathologic disorders.

CD103 dendritic cells are critical for cross-presentation of tumor antigens. Here we have shown that during immunotherapy, large numbers of cells expressing CD103 arose in murine tumors via direct differentiation of Ly6c monocytic precursors. These Ly6cCD103 cells could derive from bone-marrow monocytic progenitors (cMoPs) or from peripheral cells present within the myeloid-derived suppressor cell (MDSC) population. Differentiation was controlled by inflammation-induced activation of the transcription factor p53, which drove upregulation of Batf3 and acquisition of the Ly6cCD103 phenotype. Mice with a targeted deletion of p53 in myeloid cells selectively lost the Ly6cCD103 population and became unable to respond to multiple forms of immunotherapy and immunogenic chemotherapy. Conversely, increasing p53 expression using a p53-agonist drug caused a sustained increase in Ly6cCD103 cells in tumors during immunotherapy, which markedly enhanced the efficacy and duration of response. Thus, p53-driven differentiation of Ly6cCD103 monocytic cells represents a potent and previously unrecognized target for immunotherapy.

Indoleamine 2,3-dioxygenase (IDO) is a natural mechanism of creating acquired tolerance in a variety of physiological settings. This endogenous tolerogenic pathway has important functions in regulating the magnitude of immune responses in settings of infection, pregnancy, tissue transplantation, mucosal interfaces and others. Whether for angiogenesis, stromal formation or immunologic tolerance, tumors often rely on recruiting host mechanisms. IDO is one such potent endogenous mechanism that appears to be frequently hijacked by tumors to establish systemic immune tolerance to tumor antigens. IDO can be expressed by tumors themselves, but, in addition, its natural site of expression is the host immune cells recruited by the tumor (particularly dendritic cells and macrophages). Therapeutic strategies that target the IDO pathway have been shown to synergize with standard chemotherapy and experimental immunotherapies to break tumor-induced tolerance. When such strategies target IDO expressed in host cells, they may be able to disrupt tolerance without creating intrinsic tumor cell drug resistance.

Cells expressing indoleamine 2,3 dioxygenase (IDO) play key roles in regulating adaptive immune responses orchestrated by T cells. In this report we discuss our working model, the tryptophan depletion hypothesis, to explain links between IDO expression and inhibition of T cell responses. We posit that IDO+ cells, particularly professional antigen presenting cells (APCs) promote T cell entry but block cell cycle progression due to tryptophan catabolism. We discuss experimental evidence supporting predictions from the tryptophan depletion hypothesis and the implications that this model has for understanding the origin of tolerant states that explain immunological paradoxes, such as fetal survival, tumor persistence and failure to eradicate pathogens like HIY that cause persistent infections.

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